Deanna Mascle wrote on her blog on why all teachers should write. In it she says this about why students should write in every class every day:
Reflective writing at the beginning of a class or before a lesson can help students access existing knowledge and build a foundation for new information. Writing activities during a lesson can reinforce new knowledge and help students connect it to their existing framework. Writing after a lesson can serve multiple purposes from supporting knowledge transfer to fostering memory development to demonstrating comprehension. Plus, creative projects can increase engagement which in turn improves learning and retention of knowledge. Writing (if you do it right) is active learning. Writing (if you do it right) is fun. Writing (if you do it right) is meaningful.
See also this bit I linked to from Mascle about one year ago.
I think everyone should write. And I also think everyone should write publicly. So many are willing to write SMS messages or Facebook status updates… but what if you took just one of those ideas and fleshed them out? What if you took the time to take one of those tirades about the cost of strawberries at the local market and examined it from all perspectives; farmer, distributor, grocer, customer? By doing this you’d be teaching yourself, as you write, and if you still felt compelled enough with your argument to hit publish, everyone would be better for it.